The problem comes from DrGeo trying to create user folder in /usr/share/drgeo2.

There are a couple of files with a required RW access to fully exploit the DrGeo capability.
The architecture directory you mentioned is a good idea, in that case DrGeo just need a dedicated startup script whose main action will be to:

- ensure for a 'MyDrGeo' user folder with the following files copied (need of RW access)*, others files (ReadOnly) can be linked to the /usr/share/drgeo2: